Atomic layer deposition (ALD) is a technique that allows growth of thin films, atomic layer by layer. The technique can be illustrated with, but is not limited to, the deposition of Al2O3 from water and trimethyl aluminum (TMA) precursors. Recipes for many other materials producing insulators, metals and semiconductors, can be found in the literature. FIG. 1 schematically shows the growth of Al2O3 from water and (TMA). The general steps include: (a) insert an air hydroxilated substrate into the vacuum chamber, (b,c) The TMA precursor is pulsed and the TMA will react with the OH on the surface. TMA does not react with itself and the formed monolayer, thus passivating. (d) The unreacted TMA molecules are removed by evacuation and/or purging with an inert gas such as nitrogen or argon (e,f) Water is pulsed into the reactor. This will remove the CH3 groups, create Al—O—Al bridges and passivates the surface with Al—OH. CH4 (methane) is formed as a gaseous by-product (g) Unreacted H2O and CH4 molecules are removed by evacuation and/or purging with nitrogen. (a-g) is called a cycle and each cycle produces about 1.1 Angstrom of Al2O3. Thus, 100 cycles produces 110 Angstrom of Al2O3.
Design of ALD systems has followed different approaches, some of which are based on deposition systems used for other deposition techniques (e.g., chemical vapor deposition).
One approach is the laminar flow tube furnace, as shown in FIG. 3. In this case, a substrate 28 is inserted into a tube 24 through an access port 36. The substrate is heated using a tube furnace heater 26 and the reaction chamber inside the tube 24 is evacuated using a pump 34. The pressure is measured with a vacuum gauge 30. A continuous inert gas flow (carrier gas) is supplied from a cylinder 10 and injected into the precursor lines using inert gas lines 12. The precursors are heated using an oven 14. Precursor vapor is pulsed from precursor containers 16 and 20, using electronically controlled valves 18 and 22. These types of reactors are often found in research environments and are generally more suitable for small substrates, since large substrates increase the tube furnace size dramatically, both in diameter and length, in order to maintain sufficient temperature uniformity. The design is based on CVD systems, where very high temperatures are typically used and low temperature O-ring access is displaced from the tube furnace heater.
A second ALD system design also derived from the CVD technology is shown in FIG. 4. In this case, a shower head 42 is used to supply the precursors in an effort to uniformly disperse the chemicals over the surface. In such designs, the substrate heater is generally found inside the vacuum space. Although the showerhead vapor injection design may be effective in CVD systems, where gas or vapor is injected that only reacts at the high substrate temperature site, and where uniform gas distribution is essential for uniform film thickness, such a design can lead to clogging in ALD systems where precursor residues can react with each other at showerhead temperature. Moreover, in contrast to certain CVD processes, ALD does not typically require very uniform dispersion of the precursor do to the self-limiting nature of the process. In addition, in order to prevent condensation of precursors, the showerhead and other parts of the system are generally heated to a temperature range of 100-200° C., which can be complicated for complex geometries such as showerheads. Lastly, because of the large surface area and small cavities, the evacuation and purging of precursor in the deposition cycles can be difficult.
For production ALD systems, usually deposition stations are combined in a cluster tool arrangement. Wafers racks are inserted into a load lock, transferred in a transfer chamber through a slit valve arrangement. A robotic arm moves the substrates to the deposition station, where it is stacked vertically with other substrates using a vertical translation robot. After deposition the substrates are removed through unload-lock.
A single unit system is generally characterized by a horizontal precursor gas flow, and horizontal access port (slit valve) in the front. The height of the single unit defines the internal reactor volume and precursor flow speed, and is optimized for fast flow and gas utilization, and for one specific substrate type, thickness and diameter (usually silicon wafers).
Because of the complex nature of the design to achieve large throughput for one particular type of substrate, modification, upgrading, cleaning and repair can be very time consuming, and the systems do not lend themselves for research and development purposes.
Most ALD systems thus far have focused deposition on planar substrates such as silicon wafers, or, in the case of tube furnaces, wafer pieces, even though the ALD technique can be used to coat complex 3D structures, such as capacitor trenches, nanotubes, plastics, inverse opals, catalytic beds, photonic crystals, engine components, tools, optical parts etc. Since the ALD technology is highly scalable to large dimension samples, research of this technique in fields other than the semiconductor industry is desired and a tool that is easily adapted to a variety of samples geometries can be advantageous.